Abstract. The recent discovery of the enzyme lysine-specific demethylase 1 (LSD1) has illuminated an important cellular mechanism for epigenetic control of gene expression. In particular, dimethyl lysine 4, histone H3 (H3K4me2) is a transcription activating chromatin mark at gene promoters, and aberrant demethylation of this mark by LSD1 may broadly repress the expression of tumor suppressor genes that are important in human cancer. We and others have conducted studies verifying that LSD1 is an exciting new therapeutic target. We recently reported a series of (bis)guanidines and (bis)biguanides that are potent inhibitors of recombinant human LSD1. These inhibitors significantly increase H3K4me2 levels, initiate chromatin remodeling and induce the re-expression of tumor suppressor genes, making them suitable leads for analogue development. We were the first to demonstrate antitumor effects of LSD1 inhibitors in vitro and have recently demonstrated their significant antitumor effects in vivo. These studies provide proof or principle that inhibition of LSD1 can lead to significant antitumor effects. The central hypothesis of this proposal is that compounds that inhibit LSD1 can be identified and developed for use in the treatment of human cancer. Along these lines, the specific aims of this proposal are: Specific aim 1. Design and synthesis of multiple series of analogues as potential inhibitors of LSD1. We will use a systematic medicinal chemistry approach that includes analogue synthesis and high-throughput evaluation to produce rationally designed libraries of small-molecule LSD1 inhibitors. These analogues will be structurally related to our lead compounds, or to the LSD1 substrate. We will also use virtual screening to identify leads from commercial databases, and to suggest more potent analogues for synthesis and screening. Specific aim 2. Evaluation of newly synthesized analogues as LSD1 inhibitors and study of their epigenetic effects in cultured tumor cells. Each analogue will be evaluated as an inhibitor of purified LSD1, and the kinetics of inhibition will be determined. The cellular effects of all analogues will be monitored in the HCT116 tumor cell line. Each compound will be evaluated alone, and in combination with a DNA methyltransferase inhibitor and/or a class I/II histone deacetylase inhibitor. We will monitor specific chromatin marks and gene products to determine whether each compound causes tumor suppressor gene re-expression, and cell growth and viability will be measured. Specific aim 3. Evaluation of LSD1 inhibitors and combination treatments in vivo. Promising compounds and combination treatments will be advanced to dosing and efficacy studies in human HCT116 tumor xenografts. Using this approach, there is a high probability of identifying potent LSD1 inhibitors that have the potential to become an important new class of antitumor agent.